Increasing the Structural Span of Alkyne Metathesis

A surprising switch in absolute configuration of anti-inflammatory macrolactones

Oxacyclododecindione-type macrolactones exhibit highly potent anti-inflammatory activities even at nanomolar concentration. After the determination of the relative configuration of the stereocenters at C14 and C15 by total synthesis of 4-dechloro-14-deoxyoxacyclododecindione and 14-deoxyoxacyclododecindione, the absolute configuration has now been assigned by X-ray crystallography. Surprisingly, the absolute configuration is (14S,15R) which differs for C15 from that of the well-known derivatives of (S)-curvularin. The biological activities of both enantiomers of 14-deoxyoxacyclododecindione, obtained by racemic synthesis and optical resolution, were investigated and the ring conformation of the natural product was compared to that of (S)-curvularin and (R)-dehydrocurvularin.

Total Synthesis of (-)-Enigmazole A

A highly convergent, stereocontrolled total synthesis of the architecturally complex marine sponge metabolite (−)-enigmazole A has been achieved. Highlights include an unprecedented late-stage large-fragment Petasis–Ferrier union/rearrangement, a multicomponent Type I Anion Relay Chemistry (ARC) tactic, and a dithiane–epoxide union in conjunction with an oxazole-directed stereoselective reduction.

Total Synthesis, Stereochemical Revision, and Biological Reassessment of Mandelalide A: Chemical Mimicry of Intrafamily Relationships

Mandelalide A and three congeners had recently been isolated as the supposedly highly cytotoxic principles of an ascidian collected off the South African coastline. Since these compounds are hardly available from the natural source, a concise synthesis route was developed, targeting structure 1 as the purported representation of mandelalide A. The sequence involves an iridium-catalyzed two-directional Krische allylation and a cobalt-catalyzed carbonylative epoxide opening as entry points for the preparation of the major building blocks. The final stages feature the first implementation of terminal acetylene metathesis into natural product total synthesis, which is remarkable in that this class of substrates had been beyond the reach of alkyne metathesis for decades. Synthetic 1, however, proved not to be identical with the natural product. In an attempt to clarify this issue, NMR spectra were simulated for 20 conceivable diastereomers by using DFT followed by DP4 analysis; however, this did not provide a reliable assignment either. The puzzle was ultimately solved by the preparation of three diastereomers, of which compound 6 proved identical with mandelalide A in all analytical and spectroscopic regards. As the entire "northern sector" about the tetrahydrofuran ring in 6 shows the opposite configuration of what had originally been assigned, it is highly likely that the stereostructures of the sister compounds mandelalides B-D must be corrected analogously; we propose that these natural products are accurately represented by structures 68-70. In an attempt to prove this reassignment, an entry into mandelalides C and D was sought by subjecting an advanced intermediate of the synthesis of 6 to a largely unprecedented intramolecular Morita-Baylis-Hillman reaction, which furnished the γ-lactone derivative 74 as a mixture of diastereomers. Whereas (24R)-74 was amenable to a hydroxyl-directed dihydroxylation by using OsO4 /TMEDA as the reagent, the sister compound (24S)-74 did not follow a directed path but simply obeyed Kishi's rule; only this unexpected escape precluded the preparation of mandelalides C and D by this route. A combined spectroscopic and computational (DFT) study showed that the reasons for this strikingly different behavior of the two diastereomers of 74 are rooted in their conformational peculiarities. This aspect apart, our results show that the OsO4 /TMEDA complex reacts preferentially with electron deficient double bonds even if other alkenes are present that are more electron rich and less encumbered. Finally, in a brief biological survey authentic mandelalide A (6) was found to exhibit appreciable cytotoxicity only against one out of three tested human cancer cell lines and all synthetic congeners were hardly active. No significant fungicidal properties were observed.

Self-Assembly of Disorazole C1 through a One-Pot Alkyne Metathesis Homodimerization Strategy

Alkyne metathesis is increasingly explored as a reliable method to close macrocyclic rings, but there are no prior examples of an alkyne-metathesis-based homodimerization approach to natural products. In this approach to the cytotoxic C2-symmetric marine-derived bis(lactone) disorazole C1, a highly convergent, modular strategy is employed featuring cyclization through an ambitious one-pot alkyne cross-metathesis/ring-closing metathesis self-assembly process.

Catalytic Conia-ene and related reactions

Since its initial inception, the Conia-ene reaction, known as the intramolecular addition of enols to alkynes or alkenes, has experienced a tremendous development and appealing catalytic protocols have emerged. This review fathoms the underlying mechanistic principles rationalizing how substrate design, substrate activation, and the nature of the catalyst work hand in hand for the efficient synthesis of carbocycles and heterocycles at mild reaction conditions. Nowadays, Conia-ene reactions can be found as part of tandem reactions, and the road for asymmetric versions has already been paved. Based on their broad applicability, Conia-ene reactions have turned into a highly appreciated synthetic tool with impressive examples in natural product synthesis reported in recent years.

Self-Assembly of Disorazole C1 through a One-Pot Alkyne Metathesis Homodimerization Strategy

Alkyne metathesis is increasingly explored as a reliable method to close macrocyclic rings, but there are no prior examples of an alkyne-metathesis-based homodimerization approach to natural products. In this approach to the cytotoxic C₂ -symmetric marine-derived bis(lactone) disorazole C₁, a highly convergent, modular strategy is employed featuring cyclization through an ambitious one-pot alkyne cross-metathesis/ring-closing metathesis self-assembly process.

Total synthesis of the biphenyl alkaloid (-)-lythranidine

A sequence comprising a ring-closing alkyne metathesis of a propargyl alcohol derivative, followed by a ruthenium-catalyzed redox isomerization of the derived cycloalkyne and a transannular aza-Michael addition allowed the formation of the distinguishing piperidine-metacyclophane framework of the Lythraceum alkaloid lythanidine in a few high-yielding steps. This application attests to the excellent functional-group tolerance of a molybdenum alkylidyne complex endowed with triphenylsilanolate ligands, which enabled the macrocyclization even in the presence of protic functionalities, and thus illustrates the power of contemporary catalytic acetylene chemistry for target-oriented synthesis.

Oxygenated metabolites of n-3 polyunsaturated fatty acids as potential oxidative stress biomarkers: total synthesis of 8-F3t-IsoP, 10-F4t-NeuroP and [D4]-10-F4t-NeuroP

A wide variety of metabolic products of polyunsaturated fatty acids is of paramount importance for improving our medical knowledge in the field of oxidized lipids. Two novel metabolites of n-3 polyunsaturated fatty acids, 8-F3t-IsoP and 10-F4t-NeuroP as well as a deuterated derivative thereof were synthesized based on an acetylenic intermediate. An original approach achieved lateral chain insertion of 8-F3t-IsoP by a ring-closing alkyne metathesis/semi-reduction strategy together with a temporary tether.

From understanding to prediction: gold- and platinum-based π-acid catalysis for target oriented synthesis

During the last century, conceptual advances in organometallic chemistry were often rapidly embraced by target oriented synthesis. Feedback provided by such preparative scrutiny has greatly benefitted method development; particularly prominent are examples from the entire cross coupling arena, as well as olefin metathesis. Seen against this backdrop, it is somewhat surprising that the explosive growth of research into π-acid catalysis has not yet yielded a matching number of implementations into the synthesis of structurally complex targets of biological significance. In contrast to the massive output of methodological and mechanistic investigations, few studies illustrate the strategic use of gold, silver, or platinum catalysis in late stages of such multistep endeavors. These elaborate and highly precious compounds demand utmost confidence in the reliability and robustness of the method to be applied. In this Account, we analyze the possible reasons for this imbalance, after a short summary of the conceptual basis of carbophilic activation of π-bonds with the aid of soft transition metal cations or complexes. We pinpoint mechanistic subtleties, which, at least in part, produce a great deal of structural diversity but can jeopardize predictive power. With the advances in the understanding of π-acid catalyzed processes in general, however, this uncertainty is gradually vanishing and the entire field is transitioning from comprehension to prediction. This is expected to foster advanced applications, while recent progress in asymmetric gold catalysis further improves the preparative significance. The presented work in this Account illustrates our own commitment to the field as well as our growing confidence in the maturity of platinum and gold catalysis. The carbophilic activation of π-bonds, particularly of alkynes, provides a method to manipulate functional groups that is orthogonal to traditional carbonyl chemistry. We illustrate this notion by presenting a new approach to hydroxypyrone derivatives that has enabled the total synthesis of the fragile polyunsaturated cyclophane derivative neurymenolide A. The synthesis of the pyrrole alkaloid streptorubin by an enyne cycloisomerization is equally instructive. In addition, different manifestations of transannular hydroxyl addition reactions across alkyne partners mark the late stages of our conquests of amphidinolide F, polycavernoside A, and spirastrellolide F. Together with a few model studies and a personal selection of recent highlights from other groups, these examples augur well for future applications of π-acid catalysts in the realm of target oriented synthesis.